It's easier to always have the line in the text flow and only
conditionally insert the line in the header than to do what you ask,
which is to have conditional line in the text flow.
If the former is acceptable then the following might work if your
document structure is suitable:
place a marker containing a line at the start of the flow. each time you
place a horizontal line into the flow first place an empty marker, then
a line then re-insert the marker with a line (with a suitable
keep-property so these three items stay together).
In your header you can use the first marker on the page to draw your
"line" which will be a visible line unless the page has broken just
before the marker-line-marker combination.
David
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